X-rays are widely used in micro-analysis and imaging because of their small wavelengths and their ability to penetrate objects. Imaging applications of x-ray sources include an x-ray imaging microscope and an x-ray point projection microscope. In an x-ray imaging microscope, a characteristic line of the x-ray source (i.e., monochromatic x-rays) is typically used with an x-ray lens (such as a Fresnel lens) to image an object. The resolution and aberrations associated with an x-ray imaging microscope are usually determined by the wavelength of the characteristic line.
In contrast, in an x-ray point projection microscope, a small x-ray source is used in conjunction with geometric magnification to image an object. Because an x-ray point projection microscope does not have aberrations, the resolution of an x-ray point projection microscope is typically determined by the size of the x-ray source. Ideally, the x-ray source would be a point source. In practice, the x-ray source is considerably larger. For example, if a tungsten wire is used to provide the x-rays, the x-ray-source size may be 50-200 μm; similarly, if a dispenser cathode (such as tungsten in a calcium-oxide mixture) is used to provide the x-rays, the x-ray-source size may be 1-5 mm. These x-ray-source sizes may limit the resolution of an x-ray point projection microscope.
Moreover, in these applications there is typically a tradeoff between the x-ray intensity and the operating life of the target or the x-ray intensity and the x-ray beam quality. In particular, as the electron-beam current (and, thus, the power consumption) in an x-ray source is increased, the cross-sectional diameter of the electron beam is also increased. This usually increases the cross-sectional diameter of the beam of x-rays output by the x-ray source. Furthermore, as the electron-beam current is increased, the operating life of the target is decreased because the degradation of the location on the target that is bombarded by the electrons is accelerated.
Therefore, there is a need for an x-ray source without the problems listed above.